Afforestation of savannas in the Congolese coastal plains with eucalypt has provided wood pulp for industry and fuel energy for the local population. Typically, following afforestation, Acacia mangium are introduced to improve soil fertility and sustain productivity. Through investigations of particulate organic matter (POM), potential soil organic matter (SOM) quality was assessed in acacia and eucalypt plantations along rotations. Nutrients in POM (4000–50 μm) in the 0–5 cm soil layer were measured after five years into the second rotation (R2Y5) in relation to soil pH and P availability. Data were compared to those at the end of the first 7-year-rotation (R1Y7) and after two years into the second rotation (R2Y2) to evaluate overall SOM quality in the topsoil. At R2Y5, soil pH was higher in the pure eucalypt stands (100E) than in stands containing acacia, either in monoculture (100A) or evenly mixed with eucalypt (50A50E). Coarse POM (cPOM, 4000–250 μm) beneath 100A had the highest N concentration (1.71%), followed by those beneath 50A50E (1.42%) and 100E (1.30%). Higher N was always found in the stands containing acacia. Lower sulphur (S) concentrations and P availabilities were observed in cPOM (50A50E). The greatest amount of coarse (414.7 g) and fine (214.5 g) forest floor litter were found in 100A stands, whereas higher C concentrations were found in the 100E stands for coarse forest floor litter (36.5%) and in the 50A50E stands for fine forest floor litter (38.7%). The decrease in cPOM N and C concentrations were lower than 20% (R1Y7) and 26% (R2Y5) relative to the younger stage (R2Y2). This tendency was more pronounced in fine POM (250–50 μm) and organo-mineral fraction (< 50 μm). The main changes occurred in cPOM beneath stands containing acacia while higher weight of forest floor litter was found in 100A. Soil pH decreased in stands containing acacia. Overall N and C dynamics was enhanced in older stands (R2Y5) than in the younger stands (R2Y2). This may reveal a creation of more labile SOM with lower N and C concentrations in POM fractions in the surface layer, i.e., an ecosystem with a lower potential to mitigate climate change along rotations.

中文翻译：

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刚果沿海平原相思树和桉树人工林轮作过程中土壤有机质的质量

刚果沿海平原上的稀树草原用桉树造林为工业提供了木浆，并为当地居民提供了燃料。通常，造林后引入马占相思以提高土壤肥力并维持生产力。通过对颗粒有机物（POM）的调查，对阿拉伯树胶和桉树人工林中轮作的潜在土壤有机质（SOM）进行了评估。在第二轮（R2Y5）旋转五年后（相对于土壤pH和磷有效性），测量了0-5 cm土壤层中POM（4000-50μm）中的营养素。将数据与第一个7年轮换（R1Y7）结束时和第二年轮换（R2Y2）两年后的数据进行比较，以评估表土中的总体SOM质量。在R2Y5，无论是单一栽培（100A）还是与桉树均匀混合（50A50E），纯桉树林（100E）的土壤pH均高于含阿拉伯树胶的林木。100A以下的粗POM（cPOM，4000–250μm）具有最高的N浓度（1.71％），其次是50A50E以下的那些（1.42％）和100E以下的（1.30％）。在含金合欢的林分中总是发现较高的N。在cPOM（50A50E）中观察到较低的硫（S）浓度和P利用率。在100A的林分中发现最大数量的粗（414.7 g）和细（214.5 g）林地凋落物，而在100E的林分中有较高的C浓度（36.5％）和50A50E的林分。林地凋落物（38.7％）。相对于年轻阶段（R2Y2），cPOM N和C浓度的降低低于20％（R1Y7）和26％（R2Y5）。在细小的POM（250–50μm）和有机矿物级分（<50μm）中，这种趋势更为明显。主要变化发生在含有金合欢的林分下面的cPOM中，而在100A中发现较高的林地凋落物重量。含金合欢的林地土壤pH降低。相比于较年轻的林分（R2Y2），较老的林分（R2Y5）的总氮和碳动力学增强。这可能表明，在表层的POM馏分中具有较低的N和C浓度的SOM更加不稳定，即，生态系统在缓解气候变化方面的潜力较低。相比于较年轻的林分（R2Y2），较老的林分（R2Y5）的总氮和碳动力学增强。这可能表明在表面层的POM馏分中具有较低的N和C浓度的SOM更加不稳定，即，生态系统在缓解气候变化方面具有较低的潜力。相比于较年轻的林分（R2Y2），较老的林分（R2Y5）的总氮和碳动力学增强。这可能表明在表面层的POM馏分中具有较低的N和C浓度的SOM更加不稳定，即，生态系统在缓解气候变化方面具有较低的潜力。